An Oxford University study found that following mindfulness procedures – focusing on breathing and suspending judgment and criticism – was effective at treating depression.

Many schools encourage their pupils to practise mindfulness – but the new findings may lead to questions over whether it might be best avoided ahead of exams.

It might also be unhelpful for witnesses trying to recall whether they saw or heard something in court.

The findings, published in Psychological Science, show that participants who engaged in a 15-minute mindfulness meditation session were less able to differentiate items they actually encountered from items they only imagined.

Brent Wilson, a psychologist at the University of California, San Diego, said: ‘Our results highlight an unintended consequence of mindfulness meditation: memories may be less accurate.

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‘This is especially interesting given that previous research has primarily focused on the beneficial aspects of mindfulness training and mindfulness-based interventions.’

Mr Wilson and colleagues wonder whether the process of judgment-free thoughts and feelings might affect people’s ability to determine where a given memory came from.

For example, a real memory of eating an omelette could be confused with imagining the experience of eating an omelette. By suspending judgment, it is difficult for the mind to distinguish whether something really happened or not.

Angelina Jolie (left) and Oprah Winfrey (right) are also said to endorse the fashionable form of meditation

Mr Wilson added: ‘When memories of imagined and real experiences too closely resemble each other, people can have difficulty determining which is which, and this can lead to falsely remembering imagined experiences as actual experiences.’

In one exercise, participants in the mindfulness group were instructed to focus attention on their breathing without judgment, while those in the mind-wandering group were told to think about whatever came to mind

After the guided exercise in the first experiment, 153 participants studied a list of 15 words related to the concept of ‘trash’ such as garbage, waste, can, refuse, sewage and rubbish.

But the list did not actually include the word ‘trash’.

The results revealed that 39 per cent of the mindfulness participants then falsely recalled seeing the word ‘trash’ on the list, compared to only 20 per cent of the mind-wandering participants.

Mr Wilson said: ‘The same aspects of mindfulness that create countless benefits can also have the unintended negative consequence of increasing false-memory susceptibility.’